U.S. patent application number 14/974135 was filed with the patent office on 2017-06-22 for neutron gamma density fast neutron correction using a direct fast neutron detector.
The applicant listed for this patent is SCHLUMBERGER TECHNOLOGY CORPORATION. Invention is credited to Michael Lynn Evans.
Application Number | 20170176634 14/974135 |
Document ID | / |
Family ID | 59066152 |
Filed Date | 2017-06-22 |
United States Patent
Application |
20170176634 |
Kind Code |
A1 |
Evans; Michael Lynn |
June 22, 2017 |
Neutron Gamma Density Fast Neutron Correction Using A Direct Fast
Neutron Detector
Abstract
Methods and devices for determining accurate neutron-gamma
density (NGD) measurements of a broad range of formations. The NGD
measurements may be obtained by emitting neutrons into a formation
such that some of the neutrons inelastically scatter off elements
of the formation and generate inelastic gamma rays. Inelastic gamma
rays that return to the downhole tool may be detected.
Additionally, fast neutron signals may be directly measured with a
fast neutron detector. Some characteristics of certain formations
are believed to affect the fast neutron transport of the
formations. Thus, if a formation has one or more of such
characteristics, a correction may be applied to a count rate of
inelastic gamma rays from which the neutron-gamma density (NGD) may
be determined.
Inventors: |
Evans; Michael Lynn;
(Missouri City, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SCHLUMBERGER TECHNOLOGY CORPORATION |
Sugar Land |
TX |
US |
|
|
Family ID: |
59066152 |
Appl. No.: |
14/974135 |
Filed: |
December 18, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01V 5/104 20130101;
G01V 5/101 20130101 |
International
Class: |
G01V 5/10 20060101
G01V005/10 |
Claims
1. A method comprising: emitting neutrons into a formation using a
neutron source of a downhole tool, such that at least a portion of
the neutrons inelastically scatter off the formation to generate
inelastic gamma rays; detecting a count rate of inelastic gamma
rays using a gamma ray detector of the downhole tool; directly
measuring a fast neutron signal with a fast neutron detector of the
downhole tool that determines a count rate of fast neutrons,
wherein the fast neutron signal varies depending on a neutron
transport characteristic of the formation; determining whether the
neutron transport characteristic of the formation is expected to
cause the count rate of fast neutrons to result in a neutron gamma
density determination that is not accurate without a fast neutron
correction, wherein the fast neutron correction is not applied when
the neutron transport characteristic is not expected to cause the
count rate of fast neutrons to result in the neutron gamma density
determination that is not accurate without the fast neutron
correction; when the formation has the neutron transport
characteristic that is expected to cause the count rate of fast
neutrons to result in the neutron gamma density determination that
is not accurate without the fast neutron correction: applying the
fast neutron correction to the count rate of inelastic gamma rays,
a neutron transport correction function, or both; determining a
density of the formation based at least in part on a corrected
count rate of inelastic gamma rays, corrected neutron transport
correction function, or both; and outputting the determined density
of the formation.
2. The method of claim 1, wherein the fast neutron detector
comprises a He-4 fast neutron detector.
3. The method of claim 1, wherein determining whether the neutron
transport characteristic of the formation is expected to cause the
count rate of fast neutrons to result in the neutron gamma density
determination that is not accurate without the fast neutron
correction comprises determining whether the formation comprises a
concentration of light or heavy elements beyond a predetermined
threshold.
4. The method of claim 1, wherein determining whether the neutron
transport characteristic of the formation is expected to cause the
count rate of fast neutrons to result in the neutron gamma density
determination that is not accurate without the fast neutron
correction comprises determining whether a measured value of the
fast neutron count rate is outside a predetermined range.
5. The method of the preceding claim, wherein determining whether
the measured fast neutron count rate is outside the predetermined
range comprises: determining a non-corrected density based on a
non-corrected measured count rate of inelastic gamma rays and a
non-corrected neutron transport correction function; and comparing
the measured value of the fast neutron count rate to an expected
value of the fast neutron count rate of a formation having the
non-corrected density that is expected to cause the count rate of
fast neutrons to result in the neutron gamma density determination
that is accurate.
6. The method of the preceding claim, wherein applying the fast
neutron correction comprises using a correction function depending
on the difference between the measured value of the fast neutron
count rate and the expected value of fast neutron count rate.
7. The method of the preceding claim, wherein applying the fast
neutron correction comprises: determining the difference between
the measured value of the fast neutron count rate and the expected
value of the fast neutron count rate; and determining a correction
factor based on the correction function and the determined
difference, and correcting with the correction factor the count
rate of inelastic gamma rays, the neutron transport correction
function, or both.
8. The method of claim 1, wherein the method comprises performing
iteratively: determining whether the neutron transport
characteristic of the formation is expected to cause the count rate
of fast neutrons to result in the neutron gamma density
determination that is not accurate without the fast neutron
correction, when the formation has the neutron transport
characteristic that is expected to cause the count rate of fast
neutrons to result in the neutron gamma density determination that
is not accurate without the fast neutron correction: applying the
fast neutron correction to the count rate of an inelastic gamma
rays, a neutron transport correction function, or both; determining
a density of the formation based at least in part on the corrected
count rate of inelastic gamma rays, the neutron transport
correction function, or both; wherein an n.sup.th corrected density
determined from an n.sup.th iteration is used to determine whether
the neutron gamma density is accurate in an (n+1).sup.th iteration,
and wherein outputting the determined density comprises outputting
the density determined at the n.sup.th iteration.
9. The method of claim 1, wherein when the neutron transport
characteristic is not expected to cause the count rate of fast
neutrons to result in the neutron gamma density determination that
is not accurate without the fast neutron correction, determining
the density of the formation is based at least in part on the count
rate of inelastic gamma rays without correction, a neutron
transport function without correction, or both.
10. The method of claim 1, wherein the density of the formation is
determined at least based on the inelastic gamma-ray count rate,
the fast neutron count rate, and the neutron transport
function.
11. The method of the preceding claim, wherein the density of the
formation is determined based at least in part on the following
relationship: log ( CR .gamma. inel ) - f ( CR neutron ) - log ( C
cal N S ) c 1 = .rho. electron , ##EQU00003## where
.rho..sub.electron represents the density of the formation,
CR.sub.y.sup.inel represents the count rate of inelastic gamma
rays, CR.sub.neutron represents the count rate of fast neutrons,
f(CR.sub.neutron) represents the neutron transport correction
function, C.sub.cal represents a calibration constant, N.sub.S
represents an output of the neutron source, and c.sub.1 represents
a coefficient obtained experimentally or through nuclear modeling,
or by a combination thereof.
12. A downhole tool comprising: a neutron source configured to emit
neutrons into a formation at an energy sufficient to cause at least
a portion of the neutrons to inelastically scatter off elements of
the formation, generating inelastic gamma rays; a gamma ray
detector configured to detect a count rate of inelastic gamma rays
that scatter through the formation to reach the downhole tool; a
fast neutron detector that determines a count rate of fast
neutrons, wherein the fast neutron detector is configured to
directly measure a fast neutron signal that varies depending on a
fast neutron transport characteristic of the formation; and data
processing circuitry configured to: determine whether the neutron
transport characteristic of the formation is expected to cause the
second count rate of fast neutrons to result in a neutron gamma
density determination that is not accurate without a fast neutron
correction, wherein the fast neutron correction is not applied when
the neutron transport characteristic is not expected to cause the
count rate of neutrons to result in the neutron gamma density
determination that is not accurate without the fast neutron
correction; when the formation has the neutron transport
characteristic that is expected to cause the count rate of neutrons
to result in the neutron gamma density determination that is not
accurate without the fast neutron correction the data processing
circuitry is configured to: apply the fast neutron correction to
the count rate of inelastic gamma rays, a neutron transport
correction function, or both; determine a density of the formation
based at least in part on a corrected count rate of inelastic gamma
rays, corrected neutron transport correction function, or both; and
output the determined density of the formation.
13. The downhole tool of claim 12, wherein the fast neutron
detector comprises a He-4 fast neutron detector.
14. The downhole tool of claim 12, wherein the neutron source is a
pulsed neutron generator.
15. The downhole tool of claim 9, comprising a logging while
drilling configuration.
Description
BACKGROUND
[0001] This disclosure relates generally to neutron-gamma density
(NGD) well logging and, more particularly, to techniques for
obtaining an accurate NGD measurement in certain formations using a
correction factor based on measurements from a fast neutron
detector.
[0002] This section is intended to introduce the reader to various
aspects of art that may be related to various aspects of the
present techniques, which are described and/or claimed below. This
discussion is believed to be helpful in providing the reader with
background information to facilitate a better understanding of the
various aspects of the present disclosure. Accordingly, it should
be understood that these statements are to be read in this light,
and not as admissions of any kind.
[0003] Techniques have been developed to generate gamma rays for a
formation density measurement without radioisotopic gamma ray
sources. One such technique is referred to as a neutron-gamma
density (NGD) measurement. An NGD measurement involves emitting
neutrons into the formation using a neutron source, such as a
neutron generator. Some of these neutrons may inelastically scatter
off certain elements in the formation, generating inelastic gamma
rays that may enable a formation density determination. Although an
NGD measurement based on these gamma rays may be accurate in some
formations, the NGD measurement may be less accurate in other
formations.
SUMMARY
[0004] A summary of certain embodiments disclosed herein is set
forth below. It should be understood that these aspects are
presented merely to provide the reader with a brief summary of
these certain embodiments and that these aspects are not intended
to limit the scope of this disclosure. Indeed, this disclosure may
encompass a variety of aspects that may not be set forth below.
[0005] Embodiments of the disclosure relate to a method including
emitting neutrons into a formation using a neutron source of a
downhole so that part of the neutrons inelastically scatter off the
formation and generate inelastic gamma rays. Additionally, the
method includes detecting a count rate of inelastic gamma rays
using a gamma ray detector of the downhole tool and directly
measuring a fast neutron signal with a fast neutron detector of the
downhole tool. The fast neutron signal may vary depending on a
neutron transport characteristic of the formation. Further, the
method includes determining whether the neutron transport
characteristic of the formation is expected to cause a count rate
of neutrons to result in a neutron gamma density determination that
is not accurate without a fast neutron correction. The fast neutron
correction is not applied when the neutron transport characteristic
is not expected to cause the count rate of neutrons to result in
the neutron gamma density determination that is not accurate
without the fast neutron correction. Furthermore, when the
formation has the neutron transport characteristic that is expected
to cause the count rate of neutrons to result in the neutron gamma
density determination that is not accurate without the fast neutron
correction, the method includes applying the fast neutron
correction to the count rate of inelastic gamma rays, a neutron
transport correction function, or both. The method also includes
determining a density of the formation based at least in part on
the corrected count rate of inelastic gamma rays, the corrected
neutron transport correction function, or both and outputting the
determined density of the formation.
[0006] The fast neutron detector comprises a He-4 fast neutron
detector. However, any other appropriate fast neutron detector may
be used.
[0007] The method may comprise determining whether the neutron
transport characteristic of the formation is expected to cause the
count rate of fast neutrons to result in the neutron gamma density
determination that is not accurate without the fast neutron
correction comprises determining whether the formation comprises a
concentration of light or heavy elements beyond a predetermined
threshold and/or determining whether a measured value of the fast
neutron count rate is outside a predetermined range.
[0008] Determining whether the measured fast neutron count rate is
outside the predetermined range may comprises determining a
non-corrected density based on a non-corrected measured count rate
of inelastic gamma rays and a non-corrected neutron transport
correction function; and comparing the measured value of the fast
neutron count rate to an expected value of the fast neutron count
rate of a formation having the non-corrected density that is
expected to cause the count rate of fast neutrons to result in the
neutron gamma density determination that is accurate. In the latter
case, applying the fast neutron correction may comprise using a
correction function depending on the difference between the
measured value of the fast neutron count rate and the expected
value of fast neutron count rate. In particular, it may comprise
determining the difference between the measured value of the fast
neutron count rate and the expected value of the fast neutron count
rate; determining a correction factor based on the correction
function and the determined difference, and correcting with the
correction factor the count rate of inelastic gamma rays, the
neutron transport correction function, or both.
[0009] The method may comprise performing iteratively: [0010]
determining whether the neutron transport characteristic of the
formation is expected to cause the count rate of fast neutrons to
result in the neutron gamma density determination that is not
accurate without the fast neutron correction, [0011] when the
formation has the neutron transport characteristic that is expected
to cause the count rate of fast neutrons to result in the neutron
gamma density determination that is not accurate without the fast
neutron correction: [0012] applying the fast neutron correction to
the count rate of an inelastic gamma rays, a neutron transport
correction function, or both; [0013] determining a density of the
formation based at least in part on the corrected count rate of
inelastic gamma rays, the neutron transport correction function, or
both; an n.sup.th corrected density determined from an n.sup.th
iteration being used to determine whether the neutron gamma density
is accurate in an (n+1).sup.th iteration, and outputting the
determined density comprises outputting the density determined at
the n.sup.th iteration.
[0014] When the neutron transport characteristic is not expected to
cause the count rate of fast neutrons to result in the neutron
gamma density determination that is not accurate without the fast
neutron correction, determining the density of the formation may be
based at least in part on the count rate of inelastic gamma rays
without correction, a neutron transport function without
correction, or both.
[0015] The density of the formation may be determined at least
based on the inelastic gamma-ray count rate, the fast neutron count
rate, and the neutron transport function. In particular, the
density of the formation may be determined based at least in part
on the following relationship:
log ( CR .gamma. inel ) - f ( CR neutron ) - log ( C cal N S ) c 1
= .rho. electron , ##EQU00001##
where .rho..sub.electron represents the density of the formation,
CR.sub.y.sup.inel represents the count rate of inelastic gamma
rays, CR.sub.neutron represents the count rate of fast neutrons,
f(CR.sub.neutron) represents the neutron transport correction
function, C.sub.cal represents a calibration constant, N.sub.S
represents an output of the neutron source, and c.sub.1 represents
a coefficient obtained experimentally or through nuclear modeling,
or by a combination thereof.
[0016] In another example, a downhole tool includes a neutron
source that emits neutrons into a formation at an energy sufficient
to cause at least a portion of the neutrons to inelastically
scatter off elements of the formation, generating inelastic gamma
rays. The downhole tool also includes a gamma ray detector that
detects a count rate of inelastic gamma rays that scatter through
the formation to reach the downhole tool. Further, the downhole
tool includes a fast neutron detector that determines a count rate
of fast neutrons, and the fast neutron detector directly measures a
fast neutron signal that varies depending on a fast neutron
transport characteristic of the formation. Furthermore, the
downhole tool includes data processing circuitry that determines
whether the neutron transport characteristic of the formation is
expected to cause the second count rate of fast neutrons to result
in a neutron gamma density determination that is not accurate
without a fast neutron correction. The fast neutron correction is
not applied when the neutron transport characteristic is not
expected to cause the count rate of neutrons to result in the
neutron gamma density determination that is not accurate without
the fast neutron correction. Additionally, when the formation has
the neutron transport characteristic that is expected to cause the
count rate of neutrons to result in the neutron gamma density
determination that is not accurate without the fast neutron
correction, the data processing circuitry: applies the fast neutron
correction to the count rate of inelastic gamma rays, a neutron
transport correction function, or both; determines a density of the
formation based at least in part on a corrected count rate of
inelastic gamma rays, corrected neutron transport correction
function, or both; and outputs the determined density of the
formation.
[0017] The fast neutron detector may comprise a He-4 fast neutron
detector for instance.
[0018] The neutron source may be a pulsed neutron generator.
[0019] The downhole tool may also comprise a logging while drilling
configuration. Any other configuration, such as wireline
configuration, slickline configuration may also be provided for the
tool.
[0020] In another example, a non-transitory computer readable
medium includes executable instructions which, when executed by a
processor, cause the processor to instruct a neutron source to emit
neutrons into a formation at an energy sufficient to cause at least
a portion of the neutrons to inelastically scatter off elements of
the formation, generating inelastic gamma rays. Further, the
instructions instruct a gamma ray detector to detect a count rate
of inelastic gamma rays that scatter through the formation to reach
the downhole tool. Additionally, the instructions instruct a fast
neutron detector that determines a count rate of fast neutrons to
directly measure a fast neutron signal that varies depending on a
fast neutron transport characteristic of the formation.
Furthermore, the instructions determine whether the neutron
transport characteristic of the formation is expected to cause the
second count rate of fast neutrons to result in a neutron gamma
density determination that is not accurate without a fast neutron
correction. The fast neutron correction is not applied when the
neutron transport characteristic is not expected to cause the count
rate of neutrons to result in the neutron gamma density
determination that is not accurate without the fast neutron
correction. Additionally, when the formation has the neutron
transport characteristic that is expected to cause the count rate
of neutrons to result in the neutron gamma density determination
that is not accurate without the fast neutron correction, the
instructions: apply the fast neutron correction to the count rate
of inelastic gamma rays, a neutron transport correction function,
or both; determine a density of the formation based at least in
part on a corrected count rate of inelastic gamma rays, corrected
neutron transport correction function, or both; and output the
determined density of the formation.
[0021] Technical effects of the present disclosure include the
accurate determination of a neutron-gamma density (NGD) measurement
for a broad range of formations, including formations with light or
heavy elements. These NGD measurements may remain accurate even
when the configuration of a downhole tool used to obtain the
neutron count rates and gamma ray count rates used in the NGD
measurement does not have an optimal configuration. Further, there
is no need to dispose the fast neutron detector in the tool in an
optimal configuration either to assess the need of applying the
correction or not. Thus, an accurate NGD measurement still may be
obtained using the systems and techniques disclosed above while
enabling a flexible architecture of the tool and in particular of
the arrangement of detectors.
[0022] Various refinements of the features noted above may be
undertaken in relation to various aspects of the present
disclosure. Further features may also be incorporated in these
various aspects as well. These refinements and additional features
may exist individually or in any combination. For instance, various
features discussed below in relation to one or more of the
illustrated embodiments may be incorporated into any of the
above-described aspects of the present disclosure alone or in any
combination. The brief summary presented above is intended only to
familiarize the reader with certain aspects and contexts of
embodiments of the present disclosure without limitation to the
claimed subject matter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] Various aspects of this disclosure may be better understood
upon reading the following detailed description and upon reference
to the drawings in which:
[0024] FIG. 1 is a schematic diagram of a wellsite system employing
a neutron-gamma density (NGD) system, in accordance with an
embodiment;
[0025] FIG. 2 is a schematic section view representing an NGD
system capable of accurately measuring density in a formation that
includes light or heavy elements, in accordance with an
embodiment;
[0026] FIG. 3 is a schematic section view representing the NGD
system of FIG. 2 in a well-logging operation, in accordance with an
embodiment, wherein the plane of the section view in perpendicular
to the plane of section view of FIG. 2;
[0027] FIG. 4 is a flowchart describing an embodiment of a method
for carrying out the well-logging operation of FIG. 3;
[0028] FIG. 5 is a crossplot comparing known formation density
against formation density obtained without correcting neutron or
gamma ray count rates, in accordance with an embodiment;
[0029] FIG. 6 is a plot modeling a comparison between an He-4
reaction rate and electron density, in accordance with an
embodiment; and
[0030] FIG. 7 is a plot modeling a comparison between a fast
neutron correction ratio and effective density, in accordance with
an embodiment.
DETAILED DESCRIPTION
[0031] One or more specific embodiments of the present disclosure
will be described below. These described embodiments are only
examples of the presently disclosed techniques. Additionally, in an
effort to provide a concise description of these embodiments, all
features of an actual implementation may not be described in the
specification. It should be appreciated that in the development of
any such actual implementation, as in any engineering or design
project, numerous implementation-specific decisions must be made to
achieve the developers' specific goals, such as compliance with
system-related and business-related constraints, which may vary
from one implementation to another. Moreover, it should be
appreciated that such a development effort might be complex and
time consuming, but would nevertheless be a routine undertaking of
design, fabrication, and manufacture for those of ordinary skill
having the benefit of this disclosure.
[0032] When introducing elements of various embodiments of the
present disclosure, the articles "a," "an," and "the" are intended
to mean that there are one or more of the elements. The terms
"comprising," "including," and "having" are intended to be
inclusive and mean that there may be additional elements other than
the listed elements. Additionally, it should be understood that
references to "one embodiment" or "an embodiment" of the present
disclosure are not intended to be interpreted as excluding the
existence of additional embodiments that also incorporate the
recited features.
[0033] Embodiments of this disclosure relate to systems and
techniques for obtaining a neutron-gamma density (NGD) measurement
that is accurate for various formations including formations with
light or heavy elements. In general, a downhole tool for obtaining
such an NGD measurement may include a neutron source, at least one
neutron detector, and two gamma ray detectors. While the downhole
tool is within a borehole of a formation, the neutron source may
comprise a pulsed neutron generator emitting fast neutrons of at
least 2 MeV into the formation for a brief period of time, referred
to herein as a "burst gate," during which the neutrons may
inelastically scatter off certain elements in the formation (e.g.,
oxygen) to generate gamma rays. The gamma ray detectors of the
downhole tool may detect these inelastic gamma rays. The NGD
measurement of the formation may be a function of a count rate of
these inelastic gamma rays, corrected by a neutron transport
correction function based on a neutron count rate from the neutron
detector(s). Such a neutron transport correction function generally
may accurately account for the neutron transport of most formations
commonly encountered in an oil and/or gas well, resulting in an
accurate NGD measurement. As used herein, an "accurate" NGD
measurement may refer to an NGD measurement that is within about
0.03 g/cc the true density of a formation.
[0034] It is believed that neutron counts from some downhole tool
configurations may not accurately account for fast neutron
transport in certain formations. For instance, when the downhole
tool does not include a fast neutron detector, thermal or
epithermal neutron detectors may be used to estimate the fast
neutron distribution, but count rates from thermal or epithermal
neutron detectors may not always accurately reflect the fast
neutron transport of some formations in the same way a fast neutron
detector would. Moreover, the placement of such thermal,
epithermal, and/or fast neutron detectors in the downhole tool may
involve a variety of considerations for NGD, as well as many other
well logging measurements. As such, some of these thermal or
epithermal detectors may not be at a location within the downhole
tool that is best suited to detect count rates of neutrons so as to
accurately reflect the neutron transport of some formations, when
applied in a neutron transport correction function. These
situations may arise when an NGD measurement is obtained in certain
formations including formations with light or heavy elements beyond
some concentration limit.
[0035] The nature of these formations will now be briefly
described. As used herein, the term "formation with heavy elements"
refers to a formation with a concentration of elements of atomic
mass of 26 or greater (e.g., shales containing high concentrations
of iron or aluminum) beyond a concentration limit. The term
"formation with light elements" refers to a formation with a
concentration of elements of atomic mass less than 14 (e.g., gas,
for instance CH.sub.4) beyond a concentration limit.
[0036] According to embodiments of the present disclosure, when an
NGD measurement is obtained in a formation, having characteristics
that detectably affect the fast neutron transport in a way that
differs from other formations, the gamma ray count rate(s) used for
the NGD measurement and/or a neutron transport correction function
may be modified to more accurately account for the fast neutron
transport of the formation. These or any other suitable corrections
may be applied when the formation has one or more characteristics
that are expected to cause the count rate of neutrons and/or
neutron-induced gamma rays not to accurately correspond to a fast
neutron transport of the formation, when the count rate of neutrons
and/or gamma rays is applied in a neutron transport correction
function.
[0037] With the foregoing in mind, FIG. 1 illustrates a wellsite
system in which the disclosed NGD system can be employed. The
wellsite system of FIG. 1 may be onshore or offshore. In the
wellsite system of FIG. 1, a borehole 11 may be formed in
subsurface formations by rotary drilling using any suitable
technique. A drill string 12 may be suspended within the borehole
11 and may have a bottom hole assembly 100 that includes a drill
bit 105 at its lower end. A surface system of the wellsite system
of FIG. 1 may include a platform and derrick assembly 10 positioned
over the borehole 11, the platform and derrick assembly 10
including a rotary table 16, kelly 17, hook 18 and rotary swivel
19. The drill string 12 may be rotated by the rotary table 16,
energized by any suitable means, which engages the kelly 17 at the
upper end of the drill string 12. The drill string 12 may be
suspended from the hook 18, attached to a traveling block (not
shown), through the kelly 17 and the rotary swivel 19, which
permits rotation of the drill string 12 relative to the hook 18. A
top drive system could alternatively be used, which may be a top
drive system well known to those of ordinary skill in the art.
[0038] In the wellsite system of FIG. 1, the surface system may
also include drilling fluid or mud 26 stored in a pit 27 formed at
the well site. A pump 29 may deliver the drilling fluid 26 to the
interior of the drill string 12 via a port in the swivel 19,
causing the drilling fluid to flow downwardly through the drill
string 12 as indicated by the directional arrow 8. The drilling
fluid 26 may exit the drill string 12 via ports in the drill bit
105, and circulating upwardly through the annulus region between
the outside of the drill string 12 and the wall of the borehole 11,
as indicated by the directional arrows 9. In this way, the drilling
fluid 26 lubricates the drill bit 105 and carries formation
cuttings up to the surface, as the fluid 26 is returned to the pit
27 for recirculation.
[0039] The bottom hole assembly 100 of the wellsite system of FIG.
1 may include a logging-while-drilling (LWD) module 120 and/or a
measuring-while-drilling (MWD) module 130, a roto-steerable system
and motor 150, and the drill bit 105. The LWD module 120 can be
housed in a special type of drill collar, as is known in the art,
and can contain one or more types of logging tools. It will also be
understood that more than one LWD module can be employed, as
generally represented at numeral 120A. As such, references to the
LWD module 120 can alternatively mean a module at the position of
120A as well. The LWD module 120 may include capabilities for
measuring, processing, and storing information, as well as for
communicating with surface equipment. The LWD module 120 may be
employed to obtain a neutron-gamma density (NGD) measurement, as
will be discussed further below.
[0040] The MWD module 130 can also be housed in a special type of
drill collar, as is known in the art, and can contain one or more
devices for measuring characteristics of the drill string and drill
bit. It will also be understood that more than one MWD can be
employed, as generally represented at numeral 130A. As such,
references to the MWD module 130 can alternatively mean a module at
the position of 130A as well. The MWD module 130 may also include
an apparatus for generating electrical power to the downhole
system. Such an electrical generator may include, for example, a
mud turbine generator powered by the flow of the drilling fluid,
but other power and/or battery systems may be employed additionally
or alternatively. In the wellsite system of FIG. 1, the MWD module
130 may include one or more of the following types of measuring
devices: a weight-on-bit measuring device, a torque measuring
device, a vibration measuring device, a shock measuring device, a
stick slip measuring device, a direction measuring device, and/or
an inclination measuring device.
[0041] The LWD module 120 may be used in a neutron-gamma density
(NGD) system, as shown in FIG. 2, which can accurately measure a
density in various types of formations including formations with
light or heavy elements. It may be understood that the LWD module
120 is intended to represent one example of a general configuration
of an NGD tool, and that other suitable NGD tools may include more
or fewer components and may be configured for other means of
conveyance. Indeed, other embodiments of NGD tools employing the
general configuration of the LWD module 120 are envisaged for use
with any suitable means of conveyance, such as wireline, coiled
tubing, logging while drilling (LWD), and so forth. By way of
example, the LWD module 120 may represent a model of the
EcoScope.TM. tool by Schlumberger.
[0042] The LWD module 120 may be contained within a drill collar
202 that encircles a chassis 204 and a mud channel 205. The chassis
204 may include a variety of components used for emitting and
detecting radiation to obtain an NGD measurement. For example, a
neutron generator 206 may serve as a neutron source that emits
neutrons of at least 2 MeV, which is believed to be approximately
the minimum energy to create gamma rays through inelastic
scattering with formation elements. By way of example, the neutron
generator 206 may be an electronic neutron source, such as a
Minitron.TM. by Schlumberger Technology Corporation, which may
produce pulses of neutrons through deuteron-deuteron (d-D) and/or
deuteron-triton (d-T) reactions. Thus, the neutron generator 206
may emit neutrons around 2 MeV or 14 MeV, for example. A neutron
monitor 208 may monitor the neutron emissions from the neutron
generator 206. By way of example, the neutron monitor 208 may be a
plastic scintillator and photomultiplier that primarily detects
unscattered neutrons directly emitted from the neutron generator
206, and thus may provide a count rate signal proportional to the
neutron output rate from the rate of neutron output of the neutron
generator 206. Neutron shielding 210, which may include lead, for
example, may largely prevent neutrons from the neutron generator
206 from passing internally through the LWD module 120 toward
various radiation-detecting components on the other side of the
shielding 210.
[0043] As illustrated in FIGS. 2 and 3, the LWD module 120 may
include two near neutron detectors, namely, an epithermal neutron
detector 212 and a fast neutron detector 214. Two far thermal
neutron detectors 216A and 216B may be located at a spacing farther
from the neutron generator 206 than the neutron detectors 212 and
214. For example, the near neutron detectors 212 and 214 may be
spaced approximately 10-14 in. from the neutron generator 206, and
the far neutron detectors 216A and 216B may be spaced 18-28 in.
from the neutron generator 206. A short spacing (SS) gamma ray
detector 218 may be located between the near neutron detectors 212
and 214 and the far neutron detectors 216A and 216B. A long spacing
(LS) gamma ray detector 220 may be located beyond the far neutron
detectors 216A and 216B, at a spacing farther from the neutron
generator 206 than the gamma ray detector 218. For example, the SS
gamma ray detectors 218 may be spaced approximately 16-22 in. from
the neutron generator 206, and the LS gamma ray detector 220 may be
spaced approximately 30-38 in. from the neutron generator 206.
Alternative embodiments of the LWD module 120 may include more or
fewer of such radiation detectors, but generally may include at
least two gamma ray detectors and at least one fast neutron
detector. For instance, the fast neutron detector may be a long
spacing (LS) detector. The tool may also comprise one or more SS or
LS neutron detectors, such as an additional thermal neutron
detector. Configurations in which the tool comprises fewer
detectors than in the embodiment of FIGS. 2 and 3 are also included
in the scope of the present disclosure. The neutron detectors 212,
216A, and/or 216B may be any suitable neutron detectors.
Additionally, the fast neutron detector 214 may be any suitable
fast neutron detector, such as a He-4 fast neutron detector. Other
types of fast neutron detector, such as a plastic scintillation
detector, may be used as well. Moreover, in formations with heavy
elements, such as shales with high concentrations of iron or
aluminum, the fast neutron detector 214 may generally provide a
direct measurement for neutron flux that accurately reflects the
fast neutron transport of such formations.
[0044] Additionally, the gamma ray detectors 218 and/or 220 may be
scintillator detectors surrounded by neutron shielding. The neutron
shielding may include, for example, .sup.6Li, such as lithium
carbonate (Li.sub.2CO.sub.3), which may substantially shield the
gamma ray detectors 218 and/or 220 from thermal neutrons without
producing thermal neutron capture gamma rays. The gamma ray
detectors 218 and 220 may detect inelastic gamma rays generated
when fast neutrons from the neutron generator 206 inelastically
scatter off certain elements of a surrounding formation. As will be
discussed below, a neutron-gamma density (NGD) measurement may be a
function of the inelastic gamma ray counts obtained from the gamma
ray detectors 218 and 220, corrected for the fast neutron transport
of the formation by the direct measurement of neutron flux obtained
from the fast neutron detector 214. Using the direct measurement of
the fast neutron flux may avoid relying on an inelastic neutron
count rate that dominates a fast neutron correction calculation
that uses multiple inputs for computation. Using the systems and
techniques disclosed herein, such an NGD measurement may provide
enhanced accuracy to the system regardless of whether the formation
is a formation with a high concentration of light or heavy elements
or a formation that has one or more characteristics that may cause
the count rate of neutrons not to accurately correspond to a fast
neutron transport of the formation.
[0045] The count rates of gamma rays from the gamma ray detectors
218 and 220 and count rates of neutrons from the neutron detectors
212, 214, 216A, and/or 216B may be received by the data processing
circuitry 200 as data 222. The data processing circuitry 200 may
receive the data 222 and perform certain processing to determine
one or more properties of the surrounding formation, such as
formation density. The data processing circuitry 200 may include a
processor 224, memory 226, and/or storage 228. The processor 224
may be operably coupled to the memory 226 and/or the storage 228 to
carry out the presently disclosed techniques. These techniques may
be carried out by the processor 224 and/or other data processing
circuitry based on certain instructions executable by the processor
224. Such instructions may be stored using any suitable article of
manufacture, which may include one or more tangible,
computer-readable media to at least collectively store these
instructions. The article of manufacture may include, for example,
the memory 226 and/or the nonvolatile storage 228. The memory 226
and the nonvolatile storage 228 may include any suitable articles
of manufacture for storing data and executable instructions, such
as random-access memory, read-only memory, rewriteable flash
memory, hard drives, and optical disks.
[0046] The LWD module 120 may transmit the data 222 to the data
processing circuitry 200 via, for example, internal connections
within the tool, a telemetry system communication uplink, and/or a
communication cable. The data processing circuitry may be situated
in the tool and/or at the surface. The data processing circuitry
200 may determine one or more properties of the surrounding
formation. By way of example, such properties may include a
neutron-gamma density (NGD) measurement of the formation.
Thereafter, the data processing circuitry 200 may output a report
230 indicating the NGD measurement of the formation. The report 230
may be stored in memory or may be provided to an operator via one
or more output devices, such as an electronic display.
[0047] As shown in a neutron-gamma density (NGD) well-logging
operation 240 of FIG. 3, the LWD module 120 may be used to obtain a
neutron-gamma density (NGD) measurement that remains accurate in a
variety of formations 242. As seen in FIG. 3, the NGD well-logging
operation 240 may involve lowering the LWD module 120 into the
formation 242 through a borehole 244. In the example of FIG. 3, the
LWD module 120 can be lowered into the borehole 244 while drilling,
and thus no casing may be present in the borehole 244. However, in
other embodiments, a casing may be present. Although such casing
could attenuate a gamma-gamma density tool that utilized a gamma
ray source instead of a neutron generator 206, the presence of
casing on the borehole 244 will not prevent the determination of an
NGD measurement because neutrons 246 emitted by the neutron
generator 206 may pass through casing without significant
attenuation.
[0048] The neutron generator 206 may emit a burst of neutrons 246
for a relatively short period of time (e.g., 10 .mu.s or 20 .mu.s,
or such) sufficient to substantially only allow for inelastic
scattering to take place, referred to herein as a "burst gate." The
burst of neutrons 246 during the burst gate may be distributed
through the formation 242, the extent of which may vary depending
upon the fast neutron transport of the formation 242. For some
formations 242, counts of neutrons 246 obtained by the neutron
detectors 212, 214, 216A, and/or 216B generally may accurately
reflect the neutron transport of such formations 242. However, for
other formations 242 such as formations with light or heavy
elements, an additional correction based on a direct measure of
neutron flux may be used to more accurately account for the fast
neutron transport of the formations 242.
[0049] Many of the fast neutrons 246 emitted by the neutron
generator 206 may inelastically scatter 248 against some of the
elements of the formation 242. This inelastic scattering 248 may
produce inelastic gamma rays 250, which may be detected by the
gamma ray detectors 218 and/or 220. By determining a formation
density by taking a ratio of inelastic gamma rays 250 detected
using the two gamma ray detectors 218 and 220 at different spacings
from the neutron generator 206, lithology effects may be mostly
eliminated.
[0050] From count rates of the inelastic gamma rays 250, one or
more count rates of neutrons 246, and a determination of the
neutron output of the neutron generator 206 via the neutron monitor
208, the data processing circuitry 200 may determine an electron
density .rho..sub.electron of the formation 242. In general, the
electron density .rho..sub.electron may be calculated according to
a relationship that involves a function of a net inelastic count
rate CR.sub.y.sup.inel, corrected by a neutron transport correction
based on a direct measure of neutron flux and a downhole tool
calibration correction, which may be functions of one or more
neutron count rate(s) CR.sub.neutron and the neutron output N.sub.S
of the neutron generator 206, respectively. For example, the
electron density .rho..sub.electron calculation may take the
following form:
log ( CR .gamma. inel ) - f ( CR neutron ) - log ( C cal N S ) c 1
= .rho. electron , ( 1 ) ##EQU00002##
where CR.sub.y.sup.inel is the net inelastic gamma ray count rate
(i.e. the gamma ray count rate after subtraction of gamma rays
arising from thermal and epithermal neutron capture),
CR.sub.neutron represents a count rate of neutrons 246 from the
fast neutron detector 214, f(CR.sub.neutron) represents a neutron
transport correction, which may be a simple function of the count
rate of neutrons 246 that can correct for the fast neutron
transport of the formation 242 based on a directly measured neutron
flux, C.sub.cal is a calibration constant determined experimentally
using measurements in test formations of known composition,
porosity and density, and N.sub.S is the neutron output of the
neutron generator 206. The coefficient c.sub.1 may be determined
through characterization measurements and nuclear modeling.
[0051] For some formations 242, Equation (1) may result in an
accurate density measurement. However, for other formations
including formations 242 with relatively high concentrations of
light or heavy elements (e.g., formations 242 having concentrations
of light or heavy elements that may cause an NGD measurement to be
inaccurate without additional correction), the neutron count rate
from one or more of the neutron detectors 212, 214, 216A, and/or
216B is believed not to adequately account for the fast neutron
transport of such formations 242. Thus, when an NGD measurement is
being determined for such formations 242, the count rate of
inelastic gamma rays CR.sub.y.sup.inel, and/or the neutron
transport correction function f(CR.sub.neutron) may be corrected,
as described by a flowchart 260 of FIG. 4.
[0052] The flowchart 260 of FIG. 4 represents one embodiment of a
method for carrying out the well-logging operation 240 of FIG. 3.
While the LWD module 120 is in the borehole 244, the neutron
generator 206 may emit a burst of neutrons 246 into the formation
242 (block 262). The neutrons 246 may inelastically scatter 248 off
certain elements of the formation 242, generating inelastic gamma
rays 250. Count rate(s) of neutrons 246 as well as count rate(s) of
inelastic gamma rays 250 may be obtained (block 264). As discussed
above with reference to Equation (1), such count rate(s) of
neutrons 246 generally may relate well to the fast neutron
transport of the formation 242 for some formations 242 encountered
in an oil and/or gas well.
[0053] In other formations 242, however, it is believed that the
count rate(s) of neutrons 246 and/or the count rate(s) of gamma
rays 250 may not adequately account for the neutron transport of
such formations 242. Thus, if the data processing circuitry 200
determines that the fast neutron count rate obtained via the fast
neutron detector 214 is outside a predetermined range (decision
block 266), which indicates that the formation has characteristics
that imply need for correction, the data processing circuitry 200
may undertake a suitable correction of the count rate(s) of
inelastic gamma rays 250, and/or the neutron transport correction
function f(CR.sub.neutron), or may provide a global correction that
applies to some or all of these terms (block 268). That is, it may
be understood that modifying any of the terms in the numerator of
Equation (1) could change the resulting NGD determination.
[0054] Thus, in block 268, the data processing circuitry 200 may
undertake any suitable correction of any of the terms of Equation
(1), including the introduction of one or more additional
correction term(s), that may cause the NGD measurement to be
generally accurate for the formation 242. If the data processing
circuitry 200 does not determine that the formation 242 has such
characteristics (decision block 266), the data processing circuitry
200 may not apply such a correction. In any case, the data
processing circuitry 200 may subsequently determine an NGD
measurement of the formation 242 using the determined count rate(s)
of neutrons 246, as well as the (corrected or uncorrected) count
rate(s) of inelastic gamma rays 250, and/or the neutron transport
correction function f(CR.sub.neutron) (block 270), and output the
corrected density (block 272). By way of example, the data
processing circuitry 200 may determine the NGD measurement based on
the relationship represented by Equation (1).
[0055] As mentioned above, although an NGD measurement such as
determined using Equation (1) may accurately represent a density
measurement for some formations 242, such an NGD measurement may
not be accurate for other formations 242 such as formations having
a relatively high concentration of light or heavy elements. This
effect is apparent FIG. 5, which represents a crossplot 280
modeling the known density of a variety of types of formations 242
against an NGD measurement for the formations 242 obtained using
Equation (1) for which, for example, the count rate(s) of inelastic
gamma rays 250, and/or the neutron transport correction function
f(CR.sub.neutron) have not been corrected in the presence of, for
example, a high concentration of light or heavy elements. In the
crossplot 280, an ordinate 282 represents the logarithm of a
neutron-transport-corrected gamma ray count rate as detected by the
LS gamma ray detector 218, and an abscissa 284 represents electron
density of the formation 242 in units of g/cc. A legend indicates
various types of formations 242 that have been modeled in the
crossplot 280, including limestone, sandstone, dolomite, sandstone
with air-filled pores, alumina, sandstone with hematite, and
simulated gas. A line 286 represents an accurate correlation
between the neutron-transport-corrected gamma ray count rate and
the known formation density.
[0056] As seen in the crossplot 280, for certain formations 242,
despite variations in the densities of the formations 242, the
calculated logarithm of neutron-transport-corrected gamma ray count
rates lies along the line 286 and accurately corresponds to the
known density. These points represent the general accuracy of the
NGD determination for these formations 242. However, for formations
242 that have light 288 or heavy elements 290, the calculated
logarithm of neutron-transport-corrected gamma ray count rates lies
below and above the line 286, respectively. Since the calculated
logarithm of neutron-transport-corrected gamma ray count rates of
these formations 242 with light 288 or heavy elements 290 does not
follow the same function of change with density as the other
formations 242 (not falling along the line 286), NGD measurements
for the light element formations 288 or heavy element formations
290 obtained using the same (uncorrected) calculations as the other
formations 242 may be inaccurate.
[0057] It is believed that insufficient fast neutron transport
correction may be responsible for the inaccurate calculations for
these formations with light or heavy elements 290. Neutron
transport corrections can be obtained by modifying, for example,
the count rate(s) of inelastic gamma rays 250 and/or the neutron
transport correction function f(CR.sub.neutron) in a suitable
manner, such that the calculated logarithm of
neutron-transport-corrected gamma ray count rates of the formations
242 that have light elements 288 or heavy elements 290 are shifted
to their proper placement along the line 286.
[0058] The correction to the count rate(s) of inelastic gamma rays
250, and/or the neutron transport correction function
f(CR.sub.neutron) that is applied in block 268 of FIG. 4 may depend
on the direct measurement of the fast neutron signal. The
relationship between the direct measurement of the fast neutron
signal and the formations with heavy elements 290 may generally be
apparent, as provided in a plot 500 in FIG. 6.
[0059] In the plot 500 of FIG. 6, an ordinate 504 represents a He-4
reaction rate (response) for the fast neutron detector 214, and an
abscissa 506 represents the electron density of the formations 242,
as determined by the Equation (1), in units of g/cc. A legend
indicates various types of formations 242 that have been modeled in
the plot 500, including freshwater filled limestone, alumina,
illite, biotite, sand/hematite, kaolinite, sandstone with
gas-filled pores, and limestone with gas-filled pores. Situated
along a line 508 are the results for formations 242 of freshwater
filled limestone, while the collection of points indicated in the
plot 500 are associated with the other formations 242 indicated in
the legend. Further, line 508 serves as a reference line regarding
the application or not of the correction.
[0060] The plot 500 provides a clear separation of gas responses
518 (e.g., formation containing light elements) and shale responses
520 (e.g., formation containing heavy elements) to the He-4
reaction rate. That is, the gas responses 518 fall above the line
508 and the shale responses 520 fall below the line 508.
[0061] Such a plot may be used in the method represented by the
flowchart 260. Accordingly, determining whether a correction is
applied (block 266) may include determining a non-corrected density
with Equation (1) based on a non-corrected inelastic gamma-ray
count rate and/or a neutron transport function, and comparing the
measured value with an expected value (as given by the reference
line 508) for the determined non-corrected density. It may be
determined that the correction is applied if the difference between
the measured value and the expected value is outside a
predetermined range. Gas responses 518 and shale responses 520
generally result in the application of the correction, as described
in detail above.
[0062] Using the plot 500, the correction factors of the responses
518 and 520 are determined (block 268) using a vertical distance
between a point on the plot 500 and the line 508. Indeed, the
vertical distance corresponds to the difference between a measured
value of the fast neutron count rate and an expected value of the
fast neutron count rate for a formation that is expected to cause
the count rate of neutrons to result in an inaccurate neutron gamma
density determination (e.g., a value falling on the line).
Accordingly, the difference between the measured value of the fast
neutron count rate and the expected value of the fast neutron count
rate may be an input of a correction function for determining the
correction factor.
[0063] For example, the correction factor for a sandstone formation
with gas-filled pores 510 may be determined using a vertical
distance 512 from the formation 510 to the line 508. Similarly, the
correction factor for a kaolinite formation 514 may be determined
using a vertical distance 516 for the formation 514 to reach the
line 508. The vertical distances 512 and 516 may be positive or
negative. The correction factors determined via the vertical
distances 512 and 516 may be applied to correct the inelastic gamma
ray count rate and the neutron transport function. Subsequently,
the NGD measurement represented by Equation (1) is corrected (block
270) to remove any fast neutron transport effects, which are
generally prevalent in shales containing high concentrations of
iron, aluminum, or other heavy elements, or in gases containing
high concentrations of light elements.
[0064] The functions described by blocks 266, 268, and 270 may be
performed iteratively, with the density that was corrected in a
previous iteration is used in the current iteration for determining
whether the correction is needed. The method may stop when a
determination is made that correction is not needed (e.g., at an
iteration (n+1) when the difference between the measured value of
the fast neutron count rate and the expected value for the
corrected density is in a predetermined range). The density output
at block 272 is then the density determined at the n.sup.th
iteration.
[0065] Using a He-4 fast neutron detector in place of other
non-fast neutron detectors may provide several advantages. As
discussed in more detail below, the He-4 fast neutron detector
provides a reduction in complexity of the physics used for
determining the correction factors. That is, the He-4 fast neutron
detector provides a direct measurement of the fast neutron flux
instead of a complicated algorithm for predicting the fast neutron
flux. Additionally, the He-4 fast neutron detector may have a
greater dynamic range than other neutron detectors. Additionally,
there is a greater statistical precision associated with the
correction factors than the precision of correction factors
calculated based on other neutron detectors.
[0066] In a plot 521 of FIG. 7, which plots data using neutron
detectors other than fast neutron detectors (e.g., other than an
He-4 fast neutron detector), correction factors for gas responses
518 and shale responses 520 are again calculated using the vertical
distance between a point on the plot 521 and a line 520
representing water and oil-filled formations 242. However, in the
plot 521 of FIG. 7, an ordinate 522 representing a fast neutron
ratio and an abscissa 524 representing an effective density in g/cc
are each calculated using complicated functions of multiple
detector responses. For example, functions used to determine the
effective density and the fast neutron ratio may include many
different inputs. Accordingly, the effective density and the fast
neutron ratio are difficult to determine and parameterize even
using both measured and simulated data. Because of this, the points
on the plot 521 are determined with greater computation costs than
the direct measurements achieved using the fast neutron detector
214.
[0067] Technical effects of the present disclosure include the
accurate determination of a neutron-gamma density (NGD) measurement
for a broad range of formations, including formations with light or
heavy elements. These NGD measurements may remain accurate even
when the configurations of a downhole tool used to obtain the
neutron count rates and gamma ray count rates used in the NGD
measurement do not have optimal configurations. Thus, an accurate
NGD measurement may be obtained using the systems and techniques
disclosed above.
[0068] The specific embodiments described above have been shown by
way of example, and it should be understood that these embodiments
may be susceptible to various modifications and alternative forms.
It should be further understood that the claims are not intended to
be limited to the particular forms disclosed, but rather to cover
all modifications, equivalents, and alternatives falling within the
spirit and scope of this disclosure.
* * * * *